Gorilla tourism, widely perceived as a lucrative industry, is propelled by strong market demand with programs in five countries and for three of four gorilla subspecies. Human presence may negatively affect wild gorillas, potentially lowering immunity and increasing the likelihood of acquiring humanborne disease. Yet, behavioral impacts of humans on wild gorilla behavior remain largely unexplored, particularly for western lowland gorillas. We evaluate the impact of tourist presence, human observer numbers (tourists, trackers, and researchers), and human observer distance on the behavior of one habituated gorilla group at Bai Hokou, Central African Republic. Behavioral data were collected for more than 12 months from January 2007. Of silverback aggressive events, 39% (N5229) were human directed, but 65% were low-level soft barks. Adult females, and one in particular, were responsible for the highest number of aggressive events toward humans. Humans maintained closer proximity to the silverback when tourists were present, although tourist numbers had no significant impact on overall group activity budgets or rates of human-directed aggression. However, as research team size increased, group feeding rates decreased. Close observer–silverback distance correlated with a decrease in his feeding rates and an increase in human monitoring. He directed less aggression toward observers at distances 410 m, although observers spent 48.5% of time between 6 and 10m of the silverback. We discuss gorilla personality as a factor in human-directed aggression. We explore whether the current 7m distance limit governing gorilla tourism, based on disease transmission risks, is sufficient considering the potential behavioral stressor of close human presence. We recommend increasing minimum observation distance to 410m where possible, decreasing observer group sizes, particularly after a visit consisting of maximum numbers and restricting tourist access to 1 visit/day.

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